Process cubed watermelon, tomatoes, cucumbers, and sugar in a blender until very smooth, about 1 minute, stopping to scrape down sides as needed. Pour mixture through a fine wire-mesh strainer lined with cheesecloth into a large pitcher; discard solids.
Stir in lime juice, tequila or vodka, and, if desired, sliced jalapeño. Chill until very cold, about 6 hours. Stir well, and serve over ice. Garnish, if desired.